Periodontal diseases are bacterial-associated inflammatory diseases of the supporting tissues of the teeth and are a major public health problem. Nearly all of the human population is affected by periodontal diseases to some degree. In a recent Melbourne survey (Spencer et al., 1985) only 20% of the adult dentate sample did not require periodontal treatment while 62% required intermediate treatment and 18% required complex treatment. Brown et al. (1989), from an extensive US Dental Health survey reported that only 15% of the studied population was free of periodontal diseases. The major form of periodontal disease is gingivitis which is associated with the non-specific accumulation of dental plaque at the gingival margin. In contrast, the less prevalent, destructive form of periodontal disease (periodontitis) is associated with a subgingival infection of specific Gram-negative bacteria. Periodontitis is a major cause of tooth loss in Australian adults.
Although gingivitis may not be a necessary precondition for the development of periodontitis (Christersson et al., 1989) gingivitis is likely to predispose susceptible sites to more serious forms of periodontal disease since the specific Gram-negative bacteria that predominate in periodontitis, but which are not detectable in the healthy periodontium, have been found in low proportions in gingivitis (Moore et al., 1987). Further, the environmental conditions that develop during gingivitis are likely to favour the subsequent colonisation or growth of the species implicated in periodontitis. The control of supragingival plaque is therefore considered an important part of a preventive strategy for the control of periodontal diseases and in fact various plaque control programs have proven to be successful in the prevention of periodontal diseases (Loesche, 1976). In the majority of individuals the customary oral hygiene method of toothbrushing is usually insufficient by itself over long periods to provide a level of plaque control compatible with oral health. Consequently the incorporation of antimicrobial agents into dental products as an aid to controlling dental plaque and gingivitis has been advocated (Addy, 1988; Marsh, 1991) and is of considerable interest to toothpaste and mouthwash companies. A number of agents have been suggested as antiplaque toothpaste additives (eg. bisbiguanides, phenols, metal ions, quartenary ammonium salts) but have either negligible intra-oral activity, undesirable side-effects (eg. mucosal irritation, tooth discolouration) and/or an incompatibility with toothpaste formulations. Triclosan (2,4,4′-trichloro-2′-hydroxy diphenyl ether) an antimicrobial agent used extensively in deodorants, soaps and other dermatological preparations is currently being used as an anti-plaque toothpaste additive in some countries however there is considerable interest to find a clinically efficacious, safe and natural antiplaque agent.
Antimicrobial peptides are widely distributed in nature and play a role in the host defence of plants and animals (Boman and Hultmark, 1987; Bevins and Zasloff, 1990). They include amongst others, the amphipathic channel forming peptides, for example the cecropins isolated from the cecropia moth (Boman and Hultmark, 1987), the magainins isolated from skin secretions of the African clawed frog Xenopus laevis (Bevins and Zasloff, 1990), the dermaseptins isolated from the skin of the arboreal frog (Mor and Nicolas, 1994) and the bombinins from the skin of Bombina variegata (Simmnaco et al., 1991). Other antimicrobial peptides include the cyclic cationic peptides containing an intramolecular disulphide, for example ranalexin from bullfrog skin (Clark et al., 1994) and bactenecin from bovine neutrophils (Romeo et al., 1988). Proline-containing antimicrobial peptides also have been identified and these include the apidaecins from the lymph fluid of the honeybee (Casteels et al., 1989) and the pig myeloid antimicrobial peptide PMAP-23 (Zanetti et al., 1994).
It is now well established that the milk protein casein should be considered not only as a nutrient but also as a protecting agent against bacterial infection of the neonate mucosa as specific peptides released by tryptic or in situ digestion have been shown to possess marked biological activity. These bioactive peptides are relatively resistant to further proteolytic breakdown and have been detected in the distal portion of the small intestine and blood of humans after ingestion of cow's milk (Svedberg et al., 1985). Migliore-Samour et al. (1989) have shown that peptides β-casein (63-68) PGPIPN (SEQ ID NO:1) and αs1-casein (194-199) TTMPLW (SEQ ID NO:2) at concentrations as low as 0.1 μM exert a significant protective effect in mice against Klebsiella pneumoniae infection when injected intravenously at 0.3 nag/kg, before lethal infectious challenge. An antibacterial peptide from bovine αa2-casein (αs2-casein (fl72-203)) released by treatment of milk with glacial acetic acid has recently been characterised and shown to inhibit the growth of Escherichia coli and Staphylococuus carnosus (Zucht et al., 1995).
Antimicrobial peptides having activity against a range of Gram-positive and Gram-negative bacteria have potential in the area of oral care, functional foods, food preservatives and pharmaceuticals. Oral care products include toothpaste, mouthwash, dental floss and professionally applied materials. Functional foods include chewing gum, confectionery, breakfast cereals, infant formula, beverages, lozenges etc. Food preservatives application could include dairy products, soups, salad dressings, processed meats, baked goods, sauces etc. Pharmaceutical use would include systemic and topically applied antibiotics and anti-infectives and medications for the treatment of ulcers and other gastro-intestinal tract diseases.
For food applications, natural antimicrobials are typically used for the maintenance and extension of shelf-life in sauces, wet salads, baked goods and pastries, processed meats, refrigerated dairy products, salad dressings and soaps. Nisin has limited application as a food preservative due to a relatively narrow spectrum of antimicrobial activity and high cost. Food manufacturers using casein antimicrobial peptides as a preservative may use “all natural” label claims which are not allowed when using artificial or chemical preservatives. A major trend in the food industry is the increasing demand for low fat products which in general tend to have increased moisture levels. This creates a demand for better food preservation systems such as the incorporation of natural antimicrobials.
The global market for medications for wound healing, treatment of upper gut ulcers and inflammatory based disease represents a major pharmaceutical market. Clinicians working in the area of duodenal and gastric ulcers currently focus on the bacterium Helicobacter pylori as the causative agent in upper gut ulcers. Channel forming antimicrobial peptides that allow H+ to enter the bacterial cell have the potential for treatment of H. pylori infections by enhancing the sensitivity of the bacterium to the acid secretions of the stomach.